The name “serial peripheral interface” is used for a known serial data bus standard which permits bidirectional, synchronous and serial data transmission between a master unit and various slave units. In this context, however, an appropriate communication system comprises at least three lines between the master unit and a first slave unit for bidirectional data transmission, namely two data lines and one clock line. If there are a plurality of slave units, each of these units requires an additional select line to the master unit.